“When I first learned that plants, in a way, behaved — that they were actually communicating information to animals — my mind exploded,” said an ecologist.
As the sun rises over Kibale National Park in Uganda, red berries and orange figs hang in the rain forest’s canopy. They’re waiting for monkeys, apes or birds to scan the foliage, eat the ripe fruit and either spit or defecate seeds far from their sources, spreading their next generation to a new location.
Nearly 2,000 miles away, in the similarly mountainous rain forests of Madagascar’s Ranomafana National Park, yellow berries or fragrant, green figs await lemurs, the frugivores of this jungle. They’ll search the forest all night for their feast, later scattering the seeds.
Over millions of years of natural selection, these plants have developed ways to communicate with animals through their fruits, new research suggests, saying something like “choose me.” With traits evolved to match each animal’s sensory capacities or physical abilities, fruits can signal dinner time in the jungle, and further their plant’s survival as a species.
“When I first learned that plants, in a way, behaved — that they were actually communicating information to animals — my mind exploded,” said Kim Valenta, an evolutionary ecologist at Duke University and co-author of a study published recently in Biology Letters investigating the relationship between fruit color and animal vision.
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Valenta and her colleague Omer Nevo, an evolutionary ecologist at the University of Ulm in Germany who published another paper about scent this month in Science Advances, are working to understand how plants have adapted to appeal to animals that spread their seeds. What they are revealing is that plants deserve more credit — and that the intricate relationships between plants and animals may be critical for understanding and preserving their shared habitats.
For more than a century, biologists have wondered why fruits from closely-related plants have such different appearances, and how animals know which ones to eat.
The prevailing hypothesis has been that animals could have influenced fruit traits — like shape, location on a tree, presentation on a branch or odor and color — through natural selection. The easier it is for fruit-eaters to identify ripe fruits, the better the chance for both to survive. The animal eats, and the parent plant reproduces — by using the animals as gardeners — without lifting a root.
In a similar way, many flowers tailor their petal shape, color, texture or nectar’s scent or flavor to attract often a single pollinating species. Scientists accept that these flower traits could result from coevolution, because the relationships are so specific.
But they’ve had questions about plants reproducing through seed dispersal. How can you pin a fruit’s particular traits to an animal when many different animals, with their own evolutionary adaptations, interact with the same fruits?
Though traits like shape are easier to study, without proper tools to measure color and odor, much research has relied on human perceptions and may have overlooked how other animals may experience the world.
For the researchers, these parks in Uganda and Madagascar offered the perfect natural experiment. With similar landscapes and related plants bearing different-looking fruits that feed animals with very different sensory abilities, the researchers could reveal how animal senses may have influenced the contrasting colors of ripe fruits against foliage.
The monkeys and apes in Uganda have tricolor vision like humans, and the birds have even better vision. But most lemurs in Madagascar can only see the blue-yellow spectrum — they’re red-green colorblind, and rely more on their strong senses of smell for many behaviors.
So the researchers collected ripe and unripe fruits and foliage and analyzed their colors with a spectrometer. With a model based on the visual capacities of the seed-dispersing animals, they also determined who was most likely to detect different fruit colors contrasting against an assortment of backgrounds.
They found the colors of each fruit were optimized against their natural backdrops to meet the demands of the visual systems of their primary seed dispersers. In Uganda, fruits contrasting with foliage in colors on the red-green spectrum — say that red berry in a green cluster — popped for birds and monkeys whose eyes can see them.
But in Madagascar, fruit with blue-yellow contrasts — like those yellow berries — could be best detected by red-green colorblind lemurs (and some birds, too).
The researchers also know that lemurs dedicate large areas of the brain to olfaction, helping them communicate with one another and pick mates — often in the dark. To these excellent smellers, a fragrant brown fig might also stand out.
These results may only be extreme, localized cases supporting the hypothesis that plants and seed-dispersers evolve together. But the researchers see additional clues in a relationship between forest elephants in the Uganda park and Balanites wilsoniana, a tree that might not be able to survive without them.
When it was ripe, Valenta said she could detect the “fermented gym sock” odor of the tree’s large fruits for miles. Elephants — with huge noses and more olfactory receptor genes than any other known animal on Earth — gobble up the fallen fruits (they may make them feel tipsy too).
Only elephants can swallow the fruits and defecate the equally large seeds whole. And this plant won’t reproduce unless it passes through an elephant’s gut, Valenta said. This type of mutual dependence is seen in well-established flower-pollinator relationships, but it’s rare to find in fruit-seed-disperser relationships. Further study may offer additional evidence that animal senses influence fruit traits.
The traveler’s palm in Madagascar highlights another plant advertisement strategy targeted at animals.
In other parts of the world, related birds of paradise plants produce red or yellow coated seeds easily detectable to birds. But in Madagascar, where traveler’s palm is native, the seeds are brilliant blue and particularly detectable to aye-ayes, a kind of rat-cat-bat-looking lemur with an enhanced capacity to detect ultraviolet light, according to Valenta.
The seeds of many plants — the traveler’s palm likely included — also contain a laxative, urging whatever eats them to hastily expel them, undigested, “with a nice pile of runny fertilizer,” said Jonathan Drori, author of “Around the World in 80 Trees.”
Aye-ayes and other lemurs are also pollinators, using their strength to open the palm’s sturdy nectar packages, ensuring the plant can produce seeds in the first place.
“If something happened to those lemurs, those trees, at least in the wild, would become extinct,” Drori said.
Interactions like these remind us that plants are active members of a complicated and fragile ecosystem — not just landscape.
“We’re only just beginning to understand how much plants and animals mean to one another,” Valenta said, “which to me is just a signal that it’s more important to conserve the entire thing intact.”